The risk of theft in libraries, as in other public places, was and still is controlled and limited by means of systems generically belonging to the “electromagnetic technology”, which is able to detect when a book or another product passes through a safety gate without prior authorization.
The electromagnetic technology detects the passage of a product through a security gate, but is not able to identify the product.
In recent years, the electromagnetic systems (EM) have been overcome by the radio frequency identification systems (RFID), since the latter systems allow a greater efficiency, the best performance and an accurate identification of the product passing through.
Although a phase of replacement of said electromagnetic technology systems with the most advanced RFID technology is beginning, however a large number of systems operating with a non-RFID electromagnetic technology are now installed and used in the world.
For example, it is possible to estimate that around 200,000 non-RFID systems are now employed in the library sector.
Currently, an anti-theft door is generally formed by at least two antennas, which are integrated inside panels mounted and fixed to the ground; said panels contain the detection system.
Some labels (electronic labels in case of RFID technology or barcode labels in case of electromagnetic technology) are applied on the products to control and, if said labels are not deactivated, they triggers an alarm when passing through a security gate. On the contrary, if said labels are disabled, they will not trigger the alarm in any way.
The anti-theft security gate is normally placed at the exit of the library, of the store or, in general, of the area inside of which the products must be controlled.
The automatic detection of products by means of radio frequency identification is based on a technology that has evolved from the classical barcode and which uses radio waves to identify, locate or certify materials or objects.
When an RFID tag enters a detection area of the antenna, said tag receives, by means of magnetic induction, the energy needed to provide the information concerning recognition.
The so-called passive systems, which have no power supply inside the RFID electronic recognition label, are generally composed of two main elements:
a) a transponder (electronic label) and
b) a radio frequency reader with a corresponding digital reading/writing antenna.
The transponder or electronic label is composed in turn by an internal antenna, a silicon microchip that includes a basic modulation circuitry and a memory.
This label is coupled to the object to be controlled and recognize (such as a book, a multimedia product, i.e. CDs, DVDs, cassettes, etc, or other).
The energy required to allow the transponder to operate is provided by electromagnetic induction from a radiofrequency field called “carrier”; said field is transmitted by an RFID reader, since, passing through the loops of an antenna, an electromagnetic field generates a DC voltage.
Therefore, the information stored in the transponder (i.e. the electronic label) will be transmitted to the reader, which will be able to accurately identify the object or the product on which the label is placed.
At a frequency equal to 13.56 MHz, for example, the physical distance within which the detection can occur varies from a few mm to about 1-2 meters.
The above mentioned systems are also used with an anti-theft feature, by means of information transcribed in digital mode in the memory of the electronic label, thus allowing to receive an authorization to leave the building or to trigger the alarm in an “ON” position.
The RFID transponder is generally a small component, which is made in form of an adhesive label incorporating several components:                a silicon memory;        miniaturized electronic components for RF modulation and transmission;        metal loops, usually made of aluminum or copper, acting as an antenna.        
Said loops are made by means of special procedures in order to obtain a very thin, flexible and extremely compact device.
Generally speaking, the RFID reader is an electronic micro-controller combined with a radio frequency modulation device, which, by means of antennas, sends energy to a transponder and then reads the information received by magnetic modulation.
Said unit, also having an anti-theft feature, is able to control the digital information; in practice, the reader generates the so-called “carrier” frequency and is controlled by a computer program, which is normally installed inside the unit.
The “carrier” is therefore a radio frequency generated by the reader to transmit energy to the transponder so as to be able to read information which are subsequently re-sent by said transponder, while a periodic amplitude modulation of the “carrier” signal is used to code the transmitted data; the frequency normally used for said system is 13.56 MHz.
The RF field generated by the reader has three tasks:                generating power by electromagnetic induction in the transponder's antenna;        synchronizing the signal transmission;        recognizing the signals transmitted from the transponder.        
It is also possible to read simultaneously more transponders, which are influenced by the field of radio frequencies emitted from the reader; said multiple-reading system is known as the so-called “anti-collision” technology.
According to the known structure, an anti-theft RFID gate must be formed from at least two detection devices, which are coupled with two or more panels, placed one opposite the other, inside of which the respective antennas are inserted.
The known RFID security gates have a current flowing in the antennas (which generates an electromagnetic field) with a direction which is unchanged and allows the following options:                optimizing the parallel reading (as shown in the enclosed FIG. 2.e) with respect to the direction of the two antennas; this is obtained through the circulation of current in the antennas with the same direction (“in phase” current, FIG. 2.e);        optimizing the perpendicular reading (FIG. 2.f); this is obtained with a current passing through the antenna in the opposite direction (“counter-phase” current, FIG. 2.f).        
Therefore, known devices are able to operate very well only with respect to a specific placement of the electronic label.
In practice, the current RFID anti-theft devices have the drawback of requiring the installation of two or more panels integrating the antennas for controlling each single gate.
This drawback also implies expensive installation and system costs, since a plurality of panels of antennas must be installed.
Furthermore, the current known devices, since they require a plurality of series-connected panels, cannot be adapted to installations having different geometries.
Another limitation of the known devices is the need to provide a suitable plant for passing the connecting cables between the multiple panels of antennas constituting the detection system.
These drawbacks are particularly evident because the installation of many antennas in the opening area of the access doors frequently causes serious logistical problems.